Recently, as is known to those familiar with semiconductor microchip manufacturing technology, wafer bonding has been developed as a method for bonding large area oxidized or non-oxidized silicon wafers. Since wafer bonding makes it possible to bury oxide and implantation layers within the bulk of a monocrystalline silicon wafer, it may be a low cost and highly flexible alternative for silicon-on insulator (SOI) and epitaxial applications. Silicon direct bonding (SDB) may also lead to new device structures in the fields of power devices and sensors.
However, in spite of its apparent simplicity there is at least one major obstacle to overcome before wafer bonding can be considered as a reliable technique to produce SOI wafers for electronic devices or for other possible applications. The obstacle is bonding voids or "bubbles" which form at the interface of the mated wafers and are detrimental to the efficacy of the bond. Applicant has developed a novel and simple procedure relating to this obstacle which serves to eliminate interface bubbles at the mated wafer interface and which does not mandate that the procedure be performed in a Clean Room.
Interface bubbles are caused by dust or other particles and insufficient wafer flatness. While the latter case can be excluded by appropriate wafer specification, it is difficult to realize totally particle-free wafer surfaces prior to the bonding procedure. It has been found that, even for wafers mated in a Class 1 Clean Room, almost all of the wafers contain one or more bubbles due to enclosed particles of less than 1 micrometer (.mu.m) in size. Although the complete absence of bubbles may not be necessary for manufacturing power devices from bonded wafers, it is necessary for SOI applications and desirable for all applications. In order to produce completely void-free wafer pairs more complicated techniques rely on high pressure and annealing techniques after wafer bonding and, contrary to the findings of some researchers in this field, applicant has not found that the bubbles vanish when wafer pairs are annealed above 1000.degree. Centigrade (C.). Instead, it was found that basically all the voids introduced during the bonding process at room temperature remain during the annealing step.